The present invention relates to the field of optical position sensors and particularly to a method and an optical sensor for determining the position of a mobile body moving in a rectilinear direction and transversely with respect to a fixed reference body.
Optical sensors of this type, such as, for example, those described in the British Patent No. 956 347, are already known. These include, in particular, optical sensors comprising, mainly, three components, i.e. a radiation source integral with the fixed reference, optical detectors facing the radiation source and integral with the fixed reference, and an encoded plate integral with the mobile body, the plate being interposed and moving in the space between the source and the detectors.
The encoded plate is formed of a support transparent to light on which opaque areas, which prevent the passage of light according to a code representing the position of the mobile body, are present.
When the position of the mobile body changes, the position of the plate changes, and consequently, the intensity of the radiation received by the light receivers also changes.
Therefore, the measurement of the intensity of the radiation received by the light receivers represents the position of the mobile body.
Such known optical sensors, however, have certain disadvantages. Primarily, the encoded plate is fragile and consequently, it is easily damaged. Secondly, if the plate is subjected to spurious movements such as falling below, rising above or misalignments with respect to the mobile body, variations occur in the intensity of the radiation received by the light receivers which are not due to a variation in the position of the mobile body, and consequently, measurement of the position of the mobile body is made unreliable by such spurious movements.
Furthermore, if the light source has spurious variations in intensity, the intensity of the radiation received by the receivers also varies, even when the position of the mobile body has not changed. Also, in this case, an incorrect measurement of the position of the mobile body is obtained.
To avoid this second disadvantage, in a known solution described in the British Patent No. 2 054 135, the plate is encoded in such a way that when the plate moves and even though the intensity of the individual radiation signals received by each light receiver varies, the use of the radiation intensities received by all the receivers is independent of the position of the plate and depends only on the intensity of the radiation emitted by the source.
The intensities of the radiation received by the receivers are transformed into electrical signals, and the sum of these electrical signals (not dependent on the position of the mobile body) is compared with a fixed reference signal.
By means of a closed loop control circuit, the signal resulting from such comparison is amplified and sent to control the supply of the radiation source. In this way, if the sum of the electrical signals decreases, it means that the source is emitting a lower intensity of radiation or light than the pre-established value, and the control circuit increases the energy supply of the light source thereby causing the source to return to emitting the pre-established value of light.
In the same way, if the sum of the electric signals increases, it means that the source is emitting light at an intensity greater than that pre-established value, and the control circuit reduces the energy supply of the light source, bringing the source back to emitting light at an intensity equal to that which is pre-established.
Specifically, in said known optical sensor, it is not the determination of the position of the mobile body which is made independent of the spurious variations in the light intensity of the source, but said spurious variations are compensated by regulating the supply of the energy source in such a way that it emits light of a constant intensity.
However, in order to obtain this regulation, it was necessary to use a closed loop control circuit, and a light source energy supply which can be regulated. Both requirements cause said known position optical sensor to be high in cost and to have circuit complications.